As a piercing apparatus for piercing a billet, for example, an inclined roll type piercing apparatus, a press roll piercing apparatus, and even a piercing press are available. The inclined roll type piercing apparatus is used for the production of seamless steel pipe using the Mannesmann process. The inclined roll type piercing apparatus produces a hollow shell by piercing-rolling a round billet.
The inclined roll type piercing apparatus includes, for example, a pair of inclined rolls and a plug. The paired inclined rolls are inclined with respect to the pass line. The plug is arranged on the pass line between the paired inclined rolls. On the inclined roll type piercing apparatus, the plug is pushed into a round billet while the round billet is rotated in the circumferential direction by the inclined rolls to piercing-roll the round billet into a hollow shell.
When the round billet is piercing-rolled into a hollow shell by using the inclined roll type piercing apparatus, a flaw (hereinafter, referred to as an inner surface flaw) may occur on the inner surface of the hollow shell. The inner surface flaw occurs, for example, through the mechanism described below. During the piercing-rolling, a Mannesmann fracture occurs on the round billet, and a flaw (crack) is formed in the central portion of the transverse cross section of round billet. The flaw formed in the central portion of round billet is turned to an inner surface flaw of the hollow shell by the piercing-rolling.
If the draft ratio of a plug nose is decreased, the inner surface flaw of hollow shell caused by the Mannesmann fracture can be reduced. However, as the draft ratio of the plug nose is decreased, the thrusting ability of round billet between the inclined rolls decreases. Therefore, it is preferable that the inner surface flaw of hollow shell can be reduced by any other method.
Techniques for reducing the inner surface flaw of hollow shell have been proposed in WO 2004/052569 (Patent Literature 1) and JP2009-18338A (Patent Literature 2).
In Patent Literature 1, a plug having a specific shape is used. This plug has a front end rolling portion, a work portion, and a reeling portion. The front end rolling portion has a columnar shape having an outside diameter d, and the front end surface thereof is formed in a spherical surface shape having a radius of curvature r. The work portion is formed by an arc rotating surface having a radius of curvature R so that the work portion is continuous with the front end rolling portion and the outside diameter thereof increases toward the rear end in the axial direction. The reeling portion is formed so as to be continuous with the work portion and has a predetermined taper angle such that the outside diameter increases toward the maximum outside diameter D at the rear end in the axial direction. The outside diameter d, the radius of curvature R, the axial direction length L1 of the front end rolling portion, the axial direction length L2 of the work portion, the axial direction length L3 of the reeling portion, and the outside diameter of a billet satisfy a predetermined relational expression.
In Patent Literature 2, a pusher device having a specific construction is used. This pusher device includes a cylinder device and a pusher mandrel. The cylinder device includes a cylinder shaft. The pusher mandrel is attached to the front end of the cylinder shaft. The front end of the pusher mandrel is brought into contact with the rear end of billet. The transverse cross-sectional area of pusher mandrel and the transverse cross-sectional area of billet satisfy a predetermined relational expression. The length of pusher mandrel and the transverse cross-sectional area of pusher mandrel satisfy a predetermined relational expression. The moved distance of the front end of cylinder shaft during piercing-rolling and the outside diameter of cylinder shaft satisfy a predetermined relational expression.
In both of the techniques in Patent Literatures 1 and 2, the Mannesmann fracture can be restrained. In some cases, however, a defect is present in the center of the transverse cross section of billet before piercing-rolling. Hereinafter, such a defect is referred to as a “center defect”. The center defect is, for example, porosity or segregation occurring in the central portion of billet. The center defect includes a flaw formed in the central portion of billet. Even if the Mannesmann fracture can be restrained, if a billet having a center defect is piercing-rolled, the center defect is elongated and may appear on the inner surface of hollow shell.
Accordingly, to reduce the inner surface flaw attributable to the center defect in the billet, it is thought that the occurrence of defect is suppressed at the stage of a cast piece. For example, JP2-224856A (Patent Literature 3) discloses a technique for suppressing the occurrence of a vacancy-form defect in the central portion of the cast piece. In Patent Literature 3, before the solidification of the interior of cast piece drawn from a continuous casting mold is finished, the cast piece is forging-pressed continuously under predetermined conditions. However, it is difficult to completely eliminate the vacancy-form defect.